The Joule-Thomson Expansion

Both the compression and expansion are isobaric processes hence, the total work is given by 

the Joule-Thomson expansion is an isenthalpic process. 

Since the Joule-Thomson process is isenthalpic, the slope of each line can be represented as (dT/dp)lf. This quantity is referred to as the Joule Thomson coefficient, pj j.. Thus1 

Values for the Joule-Thomson coefficient can be obtained from equations of state. To do so, one starts with the relationship between exact differentials given by equation (1.37) to write (using molar quantities) 

nj T = 0 for the ideal gas, and the change in temperature during the Joule-Thomson expansion depends upon non-ideal behavior of the gas. 

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Figure 3.5 In the Joule-Thomson expansion, a volume of gas V, is pushed through a porous plug by a piston at pressure pt. The gas expands to a volume V2 against a second piston at a pressure p2.

The Joule-Thomson expansion can be used to liquify gases. An expansion at pressure and temperature conditions inside the dashed line envelope where /o r < 0 cools the gas. This gas is used to precool the incoming gas so that the expansion occurs at still lower temperatures. Continuing this process eventually cools the gas until it liquifies. 

The essential features of the earlier commercial models were that the two piston engines were vertically arranged inside the large finned heat exchanger in a static atmosphere of helium with LHe from the Joule-Thomson expansion collecting in the bottom of the dewar. This liquid could be either used in situ or transferred to all external storage dewar. The energy of expansion was absorbed in a crosshead on top of the dewar assembly. 

For laboratory production of LHe, other so-called Collins-type liquefiers have been built with one or two stages of GM cooling before the Joule-Thomson expansion (e.g. ref. [50]). The thermodynamic analysis of Collins helium liquefaction cycle can be found in ref. [51]. 

Let us now analyze the conditions of the Joule-Thomson expansion in more detail. From the adiabatic character ([Pg.93]

An alternative, and much more accurate, method for obtaining information on the interactions between molecules is the Joule-Thomson expansion, shown in Fig. 5. This process also forms the experimental basis for much of the science of cryogenics (the study of phenomena at low temperatures), which we will discuss in Chapter 4. Industrially, cryogenic liquids, such as liquid N2, 02, H2, and He, are produced by the Linde process, which uses Joule-Thomson expansions. N2 and 02 (and noble gases) are obtained in this process by producing and then... 

There is another mode of gaseous expansion called the Joule-Thomson expansion, in which the change in gas volume occurs at constant enthalpy AHW = 0 without any change in energy. The vector of the isenthalpic expansion then stands perpendicular to the abscissa on the ordinate and points in the negative direction (exergy consumption) as is shown in Fig. 11.10(b). 

Experiment has shown that gases become cooler during the Joule-Thomson expansion only if they are below a certain temperature known as the inversion temperature, TrThe inversion temperature is characteristic of each gas. It is related to the van der Waals constants a and b of the gas concerned by the following expression ... 

In most gases, this temperature lies within the range of ordinary temperature. Hence, they get cooled in the Joule-Thomson expansion. Hydrogen and helium, however, have very low inversion temperatures. Thus, at ordinary temperatures, these gases get warmed up instead of getting cooled in the Joule-Thomson expansion. But if hydrogen is first cooled to -80 C which is its inversion temperature and helium is first cooled to -240 C which is its inversion temperature, then these gases also get cooled on expansion in accordance with the Joule-Thomson effect. 

The Joule-Thomson expansion is a constant-enthalpy process (Section 2,12). Hence,... 

Kelvin) and performed by J. P. Joule to study departures from ideal gas behavior. The Joule-Thomson expansion, as it is called, is used in the liquefaction of gases and in refrigeration processes (see Chapter 5). ... 

That the enthalpy of the gas leaving the first cylinder is equal to that entering the second, even though the two cylinders are at different pressures, follows from the fact that the plumbing between the two can be thought of as a flow constriction, as in the Joule-Thomson expansion. Thus the analysis of Illustration 3.2-3 applies to this part of the total process. 

Flow of fluid through a constriction such as a partially open valve, a small orifice, or a porous plug (i.e., the Joule-Thomson expansion)... 

Since S. . > 0, the Joule-Thomson expansion is al.so an irreversible process. 

The enthalpy of the gas is a constant in the Joule-Thomson expansion. The measured decrease in temperature — AT and the measured decrease in pressure — Ap are combined in the ratio... 

The Joule-Thomson expansion process was introduced earlier in this book. However, the Joule-Thomson coefficient is a measure of the change in temperature due to a change in pressure. It is defined by Equation (A7). 

The Joule-Thomson expansion occurs at constant enthalpy through a valve or throttling device... 

An apparatus for the Joule-Thomson expansion of a gas. The apparatus consists of a cylinder with pistons on both ends. Between the pistons is a porous plug which allows gas molecules to pass slowly from one side to the other. By external mechanical means, the pressure on the left is maintained at some value Pj, and the pressure on the right is Pj, with Pj > Pj. The whole apparatus is insulated and does not exchange heat with the surroundings. 

We must use the real heat capacity given by Equation (5.39). During the Joule-Thomson expansion, dh is zero thus, we can rewrite the previous equation as ... 

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